Tire tread compound

ABSTRACT

The subject invention discloses a pneumatic tire having an outer circumferential tread wherein said tread is a sulfur-cured rubber composition comprised of (a) an isoprene-butadiene diblock rubber, said isoprene-butadiene diblock rubber being comprised of a butadiene block and an isoprene-butadiene block, wherein said butadiene block has a number average molecular weight which is within the range of about 25,000 to about 350,000, wherein said isoprene-butadiene block has a number average molecular weight which is within the range of about 25,000 to about 350,000, wherein said isoprene-butadiene diblock rubber has essentially one glass transition temperature which is within the range of about -100° C. to about -70° C., wherein said isoprene-butadiene diblock polymer has a Mooney ML-4 viscosity at 100° C. which is within the range of about 50 to about 140, wherein the repeat units derived from isoprene and 1,3-butadiene in the isoprene-butadiene block are in essentially random order, and wherein said isoprene-butadiene diblock polymer does not contain any blocks which are derived solely from isoprene; and (b) a second rubber selected from the group consisting of high vinyl polybutadiene rubber, styrene-isoprene-butadiene rubber, solution styrene-butadiene rubber and emulsion styrene-butadiene rubber.

This is a continuation-in-part application of U.S. patent applicationSer. No. 08/617,234, filed on Mar. 18, 1996, now pending.

BACKGROUND OF THE INVENTION

The replacement cost of tires is one of the major expenses encounteredby the trucking industry. Tire replacement cost and frequency is, ofcourse, also of concern to most automobile and light truck owners. Inrecent years, many modifications have been implemented to improve thetreadwear characteristics of tires. However, improvements in tiretreadwear characteristics have sometimes been achieved by compromisingthe traction and/or rolling resistance characteristics of the tire.

In order to reduce the rolling resistance of a tire, rubbers having ahigh rebound can be utilized in making the tires' treads. Tires madewith such rubbers undergo less energy loss during rolling and normallyalso exhibit improved treadwear characteristics. The traditional problemassociated with this approach is that the tire's wet traction and wetskid resistance characteristics are compromised. This is because goodrolling resistance which favors low energy loss and good tractioncharacteristics which favor high energy loss are viscoelasticallyinconsistent properties.

In order to balance these two viscoelastically inconsistent properties,mixtures of various types of synthetic and natural rubber are normallyutilized in tire treads. For instance, various mixtures ofstyrene-butadiene rubber and polybutadiene rubber are commonly used as arubbery material for automobile tire treads. However, such blends arenot totally satisfactory for all purposes.

U.S. Pat. No. 4,843,120 discloses that tires having improved performancecharacteristics can be prepared by utilizing rubbery polymers havingmultiple glass transition temperatures as the tread rubber. Theserubbery polymers having multiple glass transition temperatures exhibit afirst glass transition temperature which is within the range of about-110° C. to -20° C. and exhibit a second glass transition temperaturewhich is within the range of about -50° C. to 0° C. According to U.S.Pat. No. 4,843,120, these polymers are made by polymerizing at least oneconjugated diolefin monomer in a first reaction zone at a temperatureand under conditions sufficient to produce a first polymeric segmenthaving a glass transition temperature which is between -110° C. and -20°C. and subsequently continuing said polymerization in a second reactionzone at a temperature and under conditions sufficient to produce asecond polymeric segment having a glass transition temperature which isbetween -20° C. and 20° C. Such polymerizations are normally catalyzedwith an organolithium catalyst and are normally carried out in an inertorganic solvent.

U.S. Pat. No. 5,137,998 discloses a process for preparing a rubberyterpolymer of styrene, isoprene and butadiene having multiple glasstransition temperatures and having an excellent combination ofproperties for use in making tire treads which comprises terpolymerizingstyrene, isoprene and 1,3-butadiene in an organic solvent at atemperature of no more than about 40° C. in the presence of (a) at leastone member selected from the group consisting of tripiperidino phosphineoxide and alkali metal alkoxides and (b) an organolithium compound.

U.S. Pat. No. 5,047,483 discloses a pneumatic tire having an outercircumferential tread where said tread is a sulfur-cured rubbercomposition comprised of, based on 100 parts by weight rubber (phr), (A)about 10 to about 90 parts by weight of a styrene, isoprene, butadieneterpolymer rubber (SIBR), and (B) about 70 to about 30 weight percent ofat least one of cis 1,4-polyisoprene rubber and cis 1,4-polybutadienerubber wherein said SIBR rubber is comprised of (1) about 10 to about 35weight percent bound styrene, (2) about 30 to about 50 weight percentbound isoprene and (3) about 30 to about 40 weight percent boundbutadiene and is characterized by having a single glass transitiontemperature (Tg) which is in the range of about -10° C. to about -40° C.and, further the said bound butadiene structure contains about 30 toabout 40 percent 1,2-vinyl units, the said bound isoprene structurecontains about 10 to about 30 percent 3,4-units, and the sum of thepercent 1,2-vinyl units of the bound butadiene and the percent 3,4-unitsof the bound isoprene is in the range of about 40 to about 70 percent.

U.S. Pat. No. 5,272,220 discloses a styrene-isoprene-butadiene rubberwhich is particularly valuable for use in making truck tire treads whichexhibit improved rolling resistance and tread wear characteristics, saidrubber being comprised of repeat units which are derived from about 5weight percent to about 20 weight percent styrene, from about 7 weightpercent to about 35 weight percent isoprene, and from about 55 weightpercent to about 88 weight percent 1,3-butadiene, wherein the repeatunits derived from styrene, isoprene and 1,3-butadiene are inessentially random order, wherein from about 25 percent to about 40percent of the repeat units derived from the 1,3-butadiene are of thecis-microstructure, wherein from about 40 percent to about 60 percent ofthe repeat units derived from the 1,3-butadiene are of thetrans-microstructure, wherein from about 5 percent to about 25 percentof the repeat units derived from the 1,3-butadiene are of thevinyl-microstructure, wherein from about 75 percent to about 90 percentof the repeat units derived from the isoprene are of the1,4-microstructure, wherein from about 10 percent to about 25 percent ofthe repeat units derived from the isoprene are of the3,4-microstructure, wherein the rubber has a glass transitiontemperature which is within the range of about -90° C. to about -70° C.,wherein the rubber has a number average molecular weight which is withinthe range of about 150,000 to about 400,000, wherein the rubber has aweight average molecular weight of about 300,000 to about 800,000, andwherein the rubber has an inhomogeneity which is within the range ofabout 0.5 to about 1.5.

U.S. Pat. No. 5,239,009 reveals a process for preparing a rubberypolymer which comprises: (a) polymerizing a conjugated diene monomerwith a lithium initiator in the substantial absence of polar modifiersat a temperature which is within the range of about 5° C. to about 100°C. to produce a living polydiene segment having a number averagemolecular weight which is within the range of about 25,000 to about350,000; and (b) utilizing the living polydiene segment to initiate theterpolymerization of 1,3-butadiene, isoprene and styrene, wherein theterpolymerization is conducted in the presence of at least one polarmodifier at a temperature which is within the range of about 5° C. toabout 70° C. to produce a final segment which is comprised of repeatunits which are derived from 1,3-butadiene, isoprene and styrene,wherein the final segment has a number average molecular weight which iswithin the range of about 25,000 to about 350,000. The rubbery polymermade by this process is reported to be useful for improving the wet skidresistance and traction characteristics of tires without sacrificingtread wear or rolling resistance.

U.S. Pat. No. 5,061,765 discloses isoprene-butadiene copolymers havinghigh vinyl contents which can reportedly be employed in building tireswhich have improved traction, rolling resistance and abrasionresistance. These high vinyl isoprene-butadiene rubbers are synthesizedby copolymerizing 1,3-butadiene monomer and isoprene monomer in anorganic solvent at a temperature which is within the range of about -10°C. to about 100° C. in the presence of a catalyst system which iscomprised of (a) an organoiron compound, (b) an organoaluminum compound,(c) a chelating aromatic amine and (d) a protonic compound; wherein themolar ratio of the chelating amine to the organoiron compound is withinthe range of about 0.1:1 to about 1:1, wherein the molar ratio of theorganoaluminum compound to the organoiron compound is within the rangeof about 5:1 to about 200:1, and herein the molar ratio of the protoniccompound to the organoaluminum compound is within the range of about0.001:1 to about 0.2:1.

U.S. Pat. No. 5,405,927 discloses an isoprene-butadiene rubber which isparticularly valuable for use in making truck tire treads, said rubberbeing comprised of repeat units which are derived from about 20 weightpercent to about 50 weight percent isoprene and from about 50 weightpercent to about 80 weight percent 1,3-butadiene, wherein the repeatunits derived from isoprene and 1,3-butadiene are in essentially randomorder, wherein from about 3 percent to about 10 percent of the repeatunits in said rubber are 1,2-polybutadiene units, wherein from about 50percent to about 70 percent of the repeat units in said rubber are1,4-polybutadiene units, wherein from about 1 percent to about 4 percentof the repeat units in said rubber are 3,4-polyisoprene units, whereinfrom about 25 percent to about 40 percent of the repeat units in thepolymer are 1,4-polyisoprene units, wherein the rubber has a glasstransition temperature which is within the range of about -90° C. toabout -75° C., and wherein the rubber has a Mooney viscosity which iswithin the range of about 55 to about 140.

U.S. patent application Ser. No. 08/524,666, filed on Sep. 8, 1995,discloses an isoprene-butadiene diblock rubber which has an excellentcombination of properties for use in making automobile tire treads, saidisoprene-butadiene diblock rubber being comprised of a butadiene blockand an isoprene-butadiene block, wherein said butadiene block has anumber average molecular weight which is within the range of about25,000 to about 350,000, wherein said isoprene-butadiene block has anumber average molecular weight which is within the range of about25,000 to about 350,000, wherein said isoprene-butadiene diblock rubberhas a glass transition temperature which is within the range of about-100° C. to about -70° C., wherein said isoprene-butadiene diblockrubber can optionally have a second glass transition temperature whichis within the range of about -50° C. to about 0° C., wherein saidisoprene-butadiene diblock polymer has a Mooney ML-4 viscosity at 100°C. which is within the range of about 50 to about 140, and wherein therepeat units derived from isoprene and 1,3-butadiene in theisoprene-butadiene block are in essentially random order. U.S. patentapplication Ser. No. 08/524,666 further discloses tire tread compoundswhich are (1) blends of such isoprene-butadiene diblock rubbers with3,4-polyisoprene rubbers, (2) blends of such isoprene-butadiene diblockrubbers with high cis-1,4-polybutadiene rubbers and (3) blends of suchisoprene-butadiene diblock rubbers with natural rubber.

SUMMARY OF THE INVENTION

By utilizing the isoprene-butadiene diblock polymers of this inventionin tire tread compounds, treadwear characteristics can be improvedwithout compromising traction or rolling resistance. Since theisoprene-butadiene diblock polymers of this invention do not containstyrene, the cost of raw materials can also be reduced. This is becausestyrene and other vinyl aromatic monomers are expensive relative to thecost of conjugated diene monomers, such as 1,3-butadiene and isoprene.

The subject invention more specifically discloses a pneumatic tirehaving an outer circumferential tread wherein said tread is asulfur-cured rubber composition comprised of, based on 100 parts byweight of rubber, (a) from about 30 to about 80 parts of anisoprene-butadiene diblock rubber, said isoprene-butadiene diblockrubber being comprised of a butadiene block and an isoprene-butadieneblock, wherein said butadiene block has a number average molecularweight which is within the range of about 25,000 to about 350,000,wherein said isoprene-butadiene block has a number average molecularweight which is within the range of about 25,000 to about 350,000,wherein said isoprene-butadiene diblock rubber has essentially one glasstransition temperature which is within the range of about -100° C. toabout -70° C., wherein said isoprene-butadiene diblock polymer has aMooney ML-4 viscosity at 100° C. which is within the range of about 50to about 140, and wherein the repeat units derived from isoprene and1,3-butadiene in the isoprene-butadiene block are in essentially randomorder; and (b) from about 20 to about 70 parts of a second rubberselected from the group consisting of high vinyl polybutadiene rubberand styrene-isoprene-butadiene rubber.

The subject invention further reveals a pneumatic tire having an outercircumferential tread wherein said tread is a sulfur-cured rubbercomposition comprised of, based on 100 parts by weight of rubber, (a)from about 30 to about 80 parts of an isoprene-butadiene diblock rubber,said isoprene-butadiene diblock rubber being comprised of a butadieneblock and an isoprene-butadiene block, wherein said butadiene block hasa number average molecular weight which is within the range of about25,000 to about 350,000, wherein said isoprene-butadiene block has anumber average molecular weight which is within the range of about25,000 to about 350,000, wherein said isoprene-butadiene diblock rubberhas a first glass transition temperature which is within the range ofabout -100° C. to about -70° C., wherein said isoprene-butadiene diblockrubber has a second glass transition temperature which is within therange of about -50° C. to about 0° C., wherein said isoprene-butadienediblock polymer has a Mooney ML-4 at 100° C. viscosity which is withinthe range of about 50 to about 140, and wherein the repeat units derivedfrom isoprene and 1,3-butadiene in the isoprene-butadiene block are inessentially random order; and (b) from about 20 to about 70 parts of asecond rubber selected from the group consisting of high vinylpolybutadiene rubber and styrene-isoprene-butadiene rubber.

The present invention also discloses a pneumatic tire having an outercircumferential tread wherein said tread is a sulfur-cured rubbercomposition comprised of, based on 100 parts by weight of rubber, (a)from about 50 to about 75 parts of an isoprene-butadiene diblock rubber,said isoprene-butadiene diblock rubber being comprised of a butadieneblock and an isoprene-butadiene block, wherein said butadiene block hasa number average molecular weight which is within the range of about25,000 to about 350,000, wherein said isoprene-butadiene block has anumber average molecular weight which is within the range of about25,000 to about 350,000, wherein said isoprene-butadiene diblock rubberhas essentially one glass transition temperature which is within therange of about -100° C. to about -70° C., wherein saidisoprene-butadiene diblock polymer has a Mooney ML-4 viscosity at 100°C. which is within the range of about 50 to about 140, and wherein therepeat units derived from isoprene and 1,3-butadiene in theisoprene-butadiene block are in essentially random order; and (b) fromabout 25 to about 50 parts of a second rubber selected from the groupconsisting of emulsion styrene-butadiene rubber and solutionstyrene-butadiene rubber.

The subject invention further reveals a pneumatic tire having an outercircumferential tread wherein said tread is a sulfur-cured rubbercomposition comprised of, based on 100 parts by weight of rubber, (a)from about 50 to about 75 parts of an isoprene-butadiene diblock rubber,said isoprene-butadiene diblock rubber being comprised of a butadieneblock and an isoprene-butadiene block, wherein said butadiene block hasa number average molecular weight which is within the range of about25,000 to about 350,000, wherein said isoprene-butadiene block has anumber average molecular weight which is within the range of about25,000 to about 350,000, wherein said isoprene-butadiene diblock rubberhas a first glass transition temperature which is within the range ofabout -100° C. to about -70° C., wherein said isoprene-butadiene diblockrubber has a second glass transition temperature which is within therange of about -50° C. to about 0° C., wherein said isoprene-butadienediblock polymer has a Mooney ML-4 at 100° C. viscosity which is withinthe range of about 50 to about 140, and wherein the repeat units derivedfrom isoprene and 1,3-butadiene in the isoprene-butadiene block are inessentially random order; and (b) from about 25 to about 50 parts of asecond rubber selected from the group consisting of emulsionstyrene-butadiene rubber and solution styrene-butadiene rubber.

DETAILED DESCRIPTION OF THE INVENTION

The isoprene-butadiene diblock rubber (IBR) utilized in the tirecompounds of this invention is synthesized by solution polymerization.In the first step of the solution polymerization process, 1,3-butadienemonomer is polymerized to a molecular weight which is within the rangeof about 25,000 to about 350,000. The polymerization is carried out inan inert organic medium utilizing a lithium catalyst. Thispolymerization step is carried out without employing a polar modifier.It is important to conduct this polymerization step in the absence ofsignificant quantities of polar modifiers to attain the desiredmicrostructure and glass transition temperature. For example, the repeatunits which are derived from 1,3-butadiene made in the firstpolymerization step will have a low vinyl microstructure (about 6percent to about 10 percent vinyl). The polybutadiene block made in thisstep will also have a low glass transition temperature which is withinthe range of about -100° C. to about -70° C.

The inert organic medium which is utilized as the solvent will typicallybe a hydrocarbon which is liquid at ambient temperatures which can beone or more aromatic, paraffinic or cycloparaffinic compounds. Thesesolvents will normally contain from 4 to 10 carbon atoms per moleculeand will be liquids under the conditions of the polymerization. It is,of course, important for the solvent selected to be inert. The term"inert" as used herein means that the solvent does not interfere withthe polymerization reaction or react with the polymers made thereby.Some representative examples of suitable organic solvents includepentane, isooctane, cyclohexane, normal hexane, benzene, toluene,xylene, ethylbenzene and the like, alone or in admixture. Saturatedaliphatic solvents, such as cyclohexane and normal hexane, are mostpreferred.

The lithium catalysts which can be used are typically organolithiumcompounds. The organolithium compounds which are preferred can berepresented by the formula R--Li, wherein R represents a hydrocarbylradical containing from 1 to about 20 carbon atoms. Generally, suchmonofunctional organolithium compounds will contain from 1 to about 10carbon atoms. Some representative examples of organolithium compoundswhich can be employed include methyllithium, ethyllithium,isopropyllithium, n-butyllithium, sec-butyllithium, n-octyllithium,tert-octyllithium, n-decyllithium, phenyllithium, 1-napthyllithium,4-butylphenyllithium, p-tolyllithium, 1-naphthyllithium,4-butylphenyllithium, p-tolyllithium, 4-phenylbutyllithium,cyclohexyllithium, 4-butylcyclohexyllithium and4-cyclohexylbutyllithium. Organo monolithium compounds, such asalkyllithium compounds and aryllithium compounds, are usually employed.Some representative examples of preferred organo monolithium compoundsthat can be utilized include ethyllithium, isopropyllithium,n-butyllithium, secondary-butyllithium, normal-hexyllithium,tertiary-octyllithium, phenyllithium, 2-napthyllithium,4-butylphenyllithium, 4-phenylbutyllithium, cyclohexyllithium, and thelike. Normal-butyllithium and secondary-butyllithium are highlypreferred lithium initiators.

The amount of lithium catalyst utilized will vary from one organolithiumcompound to another and with the molecular weight that is desired forthe isoprene-butadiene diblock rubber being synthesized. As a generalrule in all anionic polymerizations, the molecular weight (Mooneyviscosity) of the polymer produced is inversely proportional to theamount of catalyst utilized. An amount of organolithium initiator willbe selected to result in the production of an isoprene-butadiene diblockrubber having a Mooney viscosity which is within the range of about 50to about 140. As a general rule, from about 0.01 phm (parts per hundredparts by weight of monomer) to 1 phm of the lithium catalyst will beemployed. In most cases, from 0.01 phm to 0.1 phm of the lithiumcatalyst will be employed with it being preferred to utilize 0.025 phmto 0.07 phm of the lithium catalyst.

Normally, from about 5 weight percent to about 35 weight percent of theconjugated diene monomer will be charged into the polymerization medium(based upon the total weight of the polymerization medium including theorganic solvent and monomers). In most cases, it will be preferred forthe polymerization medium to contain from about 10 weight percent toabout 30 weight percent monomers. It is typically more preferred for thepolymerization medium to contain from about 20 weight percent to about25 weight percent monomers.

The 1,3-butadiene will be polymerized at a temperature which is withinthe range of about 5° C. to about 100° C. The polymerization temperaturewill preferably be within the range of about 40° C. to about 90° C. toattain the desired microstructure for the block segment. Temperatureswithin the range of about 60° C. to about 80° C. are most preferred. Themicrostructure of the polybutadiene block segment being prepared issomewhat dependent upon the polymerization temperature.

The polymerization in the first step of the process is allowed tocontinue until essentially all of the 1,3-butadiene monomer has beenexhausted. In other words, the polymerization is allowed to run tocompletion. Since a lithium catalyst is employed to polymerize the1,3-butadiene monomer, a living polybutadiene block segment is produced.The living polybutadiene segment synthesized will have a number averagemolecular weight which is within the range of about 25,000 to about350,000.

The living polybutadiene segment will preferably have a molecular weightwhich is within the range of about 50,000 to about 200,000 and will morepreferably have a number average molecular weight which is within therange of about 70,000 to about 150,000.

The second step in the solution polymerization process involvesutilizing the living polybutadiene block segment to initiate thecopolymerization of additional 1,3-butadiene monomer and isoprenemonomer. This copolymerization is carried out in the presence of atleast one polar modifier. Ethers and tertiary amines which act as Lewisbases are representative examples of polar modifiers that can beutilized. Some specific examples of typical polar modifiers includediethyl ether, di-n-propyl ether, diisopropyl ether, di-n-butyl ether,tetrahydrofuran, dioxane, ethylene glycol dimethyl ether, ethyleneglycol diethyl ether, diethylene glycol dimethyl ether, diethyleneglycol diethyl ether, triethylene glycol dimethyl ether, trimethylamine,triethylamine, N,N,N',N'-tetramethylethylenediamine, N-methylmorpholine, N-ethyl morpholine, N-phenyl morpholine and the like.

The modifier can also be a 1,2,3-trialkoxybenzene or a1,2,4-trialkoxybenzene. Some representative examples of1,2,3-trialkoxybenzenes that can be used include1,2,3-trimethoxybenzene, 1,2,3-triethoxybenzene, 1,2,3-tributoxybenzene,1,2,3-trihexoxybenzene, 4,5,6-trimethyl-1,2,3-trimethoxybenzene,4,5,6-tri-n-pentyl-1,2,3-triethoxybenzene,5-methyl-1,2,3-trimethoxybenzene and 5-propyl-1,2,3-trimethoxybenzene.Some representative examples of 1,2,4-trialkoxybenzenes that can be usedinclude 1,2,4-trimethoxybenzene, 1,2,4-triethoxybenzene,1,2,4-tributoxybenzene, 1,2,4-tripentoxybenzene,3,5,6-trimethyl-1,2,4-trimethoxybenzene,5-propyl-1,2,4-trimethoxybenzene and3,5-dimethyl-1,2,4-trimethoxybenzene. Dipiperidinoethane,dipyrrolidinoethane, tetramethylethylene diamine, diethylene glycol,dimethyl ether and tetrahydrofuran are representative of highlypreferred modifiers. U.S. Pat. No. 4,022,959 describes the use of ethersand tertiary amines as polar modifiers in greater detail.

The utilization of 1,2,3-trialkoxybenzenes and 1,2,4-trialkoxybenzenesas modifiers is described in greater detail in U.S. Pat. No. 4,696,986.The teachings of U.S. Pat. No. 4,022,959 and U.S. Pat. No. 4,696,986 areincorporated herein by reference in their entirety. The microstructureof the repeat units which are derived from conjugated diene monomers isa function of the polymerization temperature and the amount of polarmodifier present. For example, in the polymerization of 1,3-butadiene,it is known that higher temperatures result in lower vinyl contents(lower levels of 1,2-microstructure). Accordingly, the polymerizationtemperature, quantity of modifier and specific modifier selected will bedetermined with the ultimate desired microstructure of the polymersegment being synthesized being kept in mind.

In the second step of the solution polymerization process, the finalpolymeric segment is synthesized. This is typically carried out byadding the polar modifier, additional 1,3-butadiene and isoprene to themedium containing the living polydiene segment made in the first step.This is accomplished by first adding the modifier to the mediumcontaining the living polybutadiene block and subsequently adding theisoprene, and additional 1,3-butadiene. Additional solvent can also beadded, if necessary, to maintain the total amount of monomers andpolymer within the polymerization medium within the range of about 5 toabout 35 weight percent (based upon the total weight of thepolymerization medium including monomers, polymer and solvent). It isdesirable to add a sufficient amount of solvent so as to maintain thetotal amount of polymer and monomers within the range of about 10 toabout 30 weight percent and preferably within the range of about 20 toabout 25 weight percent, based upon the total weight of the reactionmedium.

The repeat units in the final segment are, of course, derived from1,3-butadiene and isoprene. The isoprene-butadiene block will typicallybe comprised of from about 10 weight percent to about 60 weight percentrepeat units which are derived from isoprene, and from about 40 weightpercent to about 90 weight percent repeat units which are derived from1,3-butadiene. It is normally preferred for the final segment to containfrom about 20 to about 50 weight percent repeat units which are derivedfrom isoprene and from about 50 weight percent to about 80 weightpercent repeat units which are derived from 1,3-butadiene. It is mostpreferred for the final segment to contain from about 30 to about 45weight percent repeat units which are derived from isoprene and fromabout 55 weight percent to about 70 weight percent repeat units whichare derived from 1,3-butadiene.

In the second segment, the distribution of repeat units derived fromisoprene and butadiene is essentially random. The term "essentiallyrandom" as used herein means lacking a definite pattern. However, it isrealized that the concentration of repeat units derived from isopreneand butadiene may vary to some degree from one end to the other end ofthe block. The repeat units which are derived from isoprene or1,3-butadiene differ from the monomer from which they were derived inthat a double bond was consumed by the polymerization reaction.

The copolymerization of butadiene and isoprene carried out in the secondstep of this process can be conducted at the same temperature which isused in the synthesis of the first block (the polybutadiene block). Inmost cases, the second polymerization step will be conducted at aboutthe same temperature which is utilized in the first polymerization step.However, the copolymerization can be carried out at a lower temperaturewhich is within the range of about 5° C. to about 70° C. if it isdesirable to attain at higher glass transition temperature and vinylcontent for the isoprene-butadiene block.

The second polymerization step is normally allowed to continue until themonomers are exhausted. In other words, the copolymerization of1,3-butadiene and isoprene is allowed to continue until thepolymerization reaction is complete. A sufficient quantity of monomerswill be utilized to attain a number average molecular weight for thefinal segment which is within the range of about 25,000 to about350,000. It is normally preferred for the second segment to have anumber average molecular weight which is within the range of 50,000 to200,000 with number average molecular weights within the range of 70,000to 150,000 being most preferred.

The ratio of the number average molecular weight of the first segment tothe number average molecular weight of the final segment will typicallybe within the range of about 25/75 to about 75/25. This ratio plays arole in determining the morphology of the polymer and will usually bewithin the range of about 35/65 to about 65/35. The Mooney ML-4viscosity at 100° C. of the segmented rubbery polymers will generally begreater than about 50 and less than about 140. It is normally preferredfor the Mooney ML-4 viscosity at 100° C. of the rubbery diblock polymerto be within the range of 80 to 135 with Mooney ML-4 viscosities withinthe range of 100 to 130 being most preferred for oil extended rubbersbefore oil extension. The isoprene-butadiene diblock polymer can containadditional blocks which are derived from conjugated diolefin monomersand/or vinyl aromatic monomers. However, the isoprene-butadiene diblockpolymer will not contain any blocks which are derived solely fromisoprene monomer. In other words, the isoprene-butadiene diblock polymerwill not contain any polyisoprene homopolymer blocks. This is becausethe presence of blocks which contain more than about 90 percent isoprenewill compromise the desirable properties of the isoprene-butadienediblock rubber.

After the copolymerization has been completed, the isoprene-butadienediblock rubber can be recovered from the organic solvent. The diblockrubber can be recovered from the organic solvent and residue by anymeans, such as decantation, filtration and centrification. It is oftendesirable to precipitate the isoprene-butadiene diblock rubber from theorganic solvent by the addition of lower alcohols containing from about1 to about 4 carbon atoms to the polymer solution. Suitable loweralcohols for precipitation of the diblock rubber from the polymer cementinclude methanol, ethanol, isopropyl alcohol, normal-propyl alcohol andt-butyl alcohol. The utilization of lower alcohols to precipitate theisoprene-butadiene diblock rubber from the polymer cement also "kills"the living polymer by inactivating lithium end groups. After the diblockrubber is recovered from the solution, steam stripping can be employedto reduce the level of volatile organic compounds in the diblock rubber.

There are valuable benefits associated with utilizing theisoprene-butadiene diblock rubbers of this invention in making tiretread compounds. Tire tread compounds can be made using these diblockrubbers without the need to blend additional rubbers therein. However,in many cases, it will be desirable to blend the isoprene-butadienediblock rubber with one or more additional rubbers to attain the desiredperformance characteristics for the tire tread compound.

The isoprene-butadiene diblock rubbers of this invention can becompounded utilizing conventional ingredients and standard techniques.For instance, the isoprene-butadiene diblock rubbers will typically beblended with carbon black and/or silica, sulfur, additional fillers,accelerators, oils, waxes, scorch inhibiting agents, coupling agents andprocessing aids. In most cases, the isoprene-butadiene diblock rubberwill be compounded with sulfur and/or a sulfur containing compound, atleast one filler, at least one accelerator, at least one antidegradant,at least one processing oil, zinc oxide, optionally a tackifier resin,optionally a reinforcing resin, optionally one or more fatty acids,optionally a peptizer and optionally one or more scorch inhibitingagents. Such blends will normally contain from about 0.5 to 5 phr (partsper hundred parts of rubber by weight) of sulfur and/or a sulfurcontaining compound with 1 phr to 2.5 phr being preferred. It may bedesirable to utilize insoluble sulfur in cases where bloom is a problem.

Normally from 10 to 150 phr of at least one filler will be utilized inthe blend with 30 to 80 phr being preferred. In most cases, at leastsome carbon black will be utilized in the filler. The filler can, ofcourse, be comprised totally of carbon black. Silica can be included inthe filler to improve tear resistance and heat build-up. Clays and/ortalc can be included in the filler to reduce cost. The blend will alsonormally include from 0.1 to 2.5 phr of at least one accelerator with0.2 to 1.5 phr being preferred. Antidegradants, such as antioxidants andantiozonants, will generally be included in the tread compound blend inamounts ranging from 0.25 to 10 phr with amounts in the range of 1 to 5phr being preferred. Processing oils will generally be included in theblend in amounts ranging from 2 to 100 phr with amounts ranging from 5to 50 phr being preferred. The IBR containing blends of this inventionwill also normally contain from 0.5 to 10 phr of zinc oxide with 1 to 5phr being preferred. These blends can optionally contain from 0 to 10phr of tackifier resins, 0 to 10 phr of reinforcing resins, 1 to 10 phrof fatty acids, 0 to 2.5 phr of peptizers, and 0 to 1 phr of scorchinhibiting agents.

To fully realize the total advantages of the blends of this invention,silica can be included in the tread rubber formulation. The processingof the rubber blend is normally conducted in the presence of a sulfurcontaining organosilicon compound to realize maximum benefits. Examplesof suitable sulfur containing organosilicon compounds are of theformula:

    Z-Alk-S.sub.n -Alk-Z                                       (I)

in which Z is selected from the group consisting of ##STR1## where R¹ isan alkyl group of 1 to 4 carbon atoms, cyclohexyl or phenyl; wherein R²is alkoxy of 1 to 8 carbon atoms, or cycloalkoxy of 5 to 8 carbon atoms;and wherein Alk is a divalent hydrocarbon of 1 to 18 carbon atoms and nis an integer of 2 to 8.

Specific examples of sulfur containing organosilicon compounds which maybe used in accordance with the present invention include:3,3'-bis(trimethoxysilylpropyl) disulfide,3,3'-bis(triethoxysilylpropyl) tetrasulfide,3,3'-bis(triethoxysilylpropyl) octasulfide,3,3'-bis(trimethoxysilylpropyl) tetrasulfide,2,2'-bis(triethoxysilylethyl) tetrasulfide,3,3'-bis(trimethoxysilylpropyl) trisulfide,3,3'-bis(triethoxysilylpropyl) trisulfide,3,3'-bis(tributoxysilylpropyl) disulfide,3,3'-bis(trimethoxysilylpropyl) hexasulfide,3,3'-bis(trimethoxysilylpropyl) octasulfide,3,3'-bis(trioctoxysilylpropyl) tetrasulfide,3,3'-bis(trihexoxysilylpropyl) disulfide,3,3'-bis(tri-2"-ethylhexoxysilylpropyl) trisulfide,3,3'-bis(triisooctoxysilylpropyl) tetrasulfide,3,3'-bis(tri-t-butoxysilylpropyl) disulfide, 2,2'-bis(methoxy diethoxysilyl ethyl) tetrasulfide, 2,2'-bis(tripropoxysilylethyl) pentasulfide,3,3'-bis(tricyclonexoxysilylpropyl) tetrasulfide,3,3'-bis(tricyclopentoxysilylpropyl) trisulfide,2,2'-bis(tri-2"-methylcyclohexoxysilylethyl) tetrasulfide,bis(trimethoxysilylmethyl) tetrasulfide, 3-methoxy ethoxy propoxysilyl3'-diethoxybutoxysilylpropyltetrasulfide, 2,2'-bis(dimethylmethoxysilylethyl) disulfide, 2,2'-bis(dimethyl sec.butoxysilylethyl)trisulfide, 3,3'-bis(methyl butylethoxysilylpropyl) tetrasulfide,3,3'-bis(di t-butylmethoxysilylpropyl) tetrasulfide, 2,2'-bis(phenylmethyl methoxysilylethyl) trisulfide, 3,3'-bis(diphenylisopropoxysilylpropyl) tetrasulfide, 3,3'-bis(diphenylcyclohexoxysilylpropyl) disulfide, 3,3'-bis(dimethylethylmercaptosilylpropyl) tetrasulfide, 2,2'-bis(methyldimethoxysilylethyl) trisulfide, 2,2'-bis(methylethoxypropoxysilylethyl) tetrasulfide, 3,3'-bis(diethylmethoxysilylpropyl) tetrasulfide, 3,3'-bis(ethyl di-sec.butoxysilylpropyl) disulfide, 3,3'-bis(propyl diethoxysilylpropyl)disulfide, 3,3'-bis(butyl dimethoxysilylpropyl) trisulfide,3,3'-bis(phenyl dimethoxysilylpropyl) tetrasulfide, 3-phenylethoxybutoxysilyl 3'-trimethoxysilylpropyl tetrasulfide,4,4'-bis(trimethoxysilylbutyl) tetrasulfide,6,6'-bis(triethoxysilylhexyl) tetrasulfide,12,12'-bis(triisopropoxysilyl dodecyl) disulfide,18,18'-bis(trimethoxysilyloctadecyl) tetrasulfide,18,18'-bis(tripropoxysilyloctadecenyl) tetrasulfide,4,4'-bis(trimethoxysilyl-buten-2-yl) tetrasulfide,4,4'-bis(trimethoxysilylcyclohexylene) tetrasulfide,5,5'-bis(dimethoxymethylsilylpentyl) trisulfide,3,3'-bis(trimethoxysilyl-2-methylpropyl) tetrasulfide and3,3'-bis(dimethoxyphenylsilyl-2-methylpropyl) disulfide.

The preferred sulfur containing organosilicon compounds are the3,3'-bis(trimethoxy or triethoxy silylpropyl) sulfides. The mostpreferred compound is 3,3'-bis(triethoxysilylpropyl) tetrasulfide.Therefore as to formula I, preferably Z is ##STR2## where R² is analkoxy of 2 to 4 carbon atoms, with 2 carbon atoms being particularlypreferred; Alk is a divalent hydrocarbon of 2 to 4 carbon atoms with 3carbon atoms being particularly preferred; and n is an integer of from 3to 5 with 4 being particularly preferred.

The amount of the sulfur containing organosilicon compound of formula Iin a rubber composition will vary depending on the level of silica thatis used. Generally speaking, the amount of the compound of formula Iwill range from about 0.01 to about 1.0 parts by weight per part byweight of the silica. Preferably, the amount will range from about 0.02to about 0.4 parts by weight per part by weight of the silica. Morepreferably, the amount of the compound of formula I will range fromabout 0.05 to about 0.25 parts by weight per part by weight of thesilica.

In addition to the sulfur containing organosilicon, the rubbercomposition should contain a sufficient amount of silica and carbonblack, if used, to contribute a reasonably high modulus and highresistance to tear. The silica filler may be added in amounts rangingfrom about 10 phr to about 250 phr. Preferably, the silica is present inan amount ranging from about 50 phr to about 120 phr. If carbon black isalso present, the amount of carbon black, if used, may vary. Generallyspeaking, the amount of carbon black will vary from about 5 phr to about80 phr. Preferably, the amount of carbon black will range from about 10phr to about 40 phr. It is to be appreciated that the silica coupler maybe used in conjunction with a carbon black, namely pre-mixed with acarbon black prior to addition to the rubber composition, and suchcarbon black is to be included in the aforesaid amount of carbon blackfor the rubber composition formulation. In any case, the total quantityof silica and carbon black will be at least about 30 phr. The combinedweight of the silica and carbon black, as hereinbefore referenced, maybe as low as about 30 phr, but is preferably from about 45 to about 130phr.

The commonly employed siliceous pigments used in rubber compoundingapplications can be used as the silica in this invention, includingpyrogenic and precipitated siliceous pigments (silica), althoughprecipitate silicas are preferred. The siliceous pigments preferablyemployed in this invention are precipitated silicas such as, forexample, those obtained by the acidification of a soluble silicate,e.g., sodium silicate.

Such silicas might be characterized, for example, by having a BETsurface area, as measured using nitrogen gas, preferably in the range ofabout 40 to about 600, and more usually in a range of about 50 to about300 square meters per gram. The BET method of measuring surface area isdescribed in the Journal of the American Chemical Society, Volume 60,page 304 (1930).

The silica may also be typically characterized by having adibutylphthalate (DBP) absorption value in a range of about 100 to about400, and more usually about 150 to about 300. The silica might beexpected to have an average ultimate particle size, for example, in therange of 0.01 to 0.05 micron as determined by the electron microscope,although the silica particles may be even smaller, or possibly larger,in size.

Various commercially available silicas may be considered for use in thisinvention such as, only for example herein, and without limitation,silicas commercially available from PPG Industries under the Hi-Siltrademark with designations 210, 243, etc; silicas available fromRhone-Poulenc, with, for example, designation of Z1165MP and silicasavailable from Degussa AG with, for example, designations VN2 and VN3.

Tire tread formulations which include silica and an organosiliconcompound can be mixed utilizing a thermomechanical mixing technique toattain a better balance of tread compound performance characteristics,for example, traction, treadwear and rolling resistance characteristics.On the other hand, the mixing of the tire tread rubber formulation canbe accomplished by conventional methods known to those having skill inthe rubber mixing art. For example, the ingredients are typically mixedin at least two stages, namely at least one non-productive stagefollowed by a productive mix stage. The final curatives including sulfurvulcanizing agents are typically mixed in the final stage which isconventionally called the "productive" mix stage in which the mixingtypically occurs at a temperature, or ultimate temperature, lower thanthe mix temperature(s) than the preceding non-productive mix stage(s).The rubber, silica and sulfur containing organosilicon, and carbon blackif used, are mixed in one or more non-productive mix stages. The terms"non-productive" and "productive" mix stages are well known to thosehaving skill in the rubber mixing art. In typical non-productive mixingprocedures, the mixing is carried out over a total mixing period of onlyone to three minutes with the rubber mixture being discharged from themixing equipment at a temperature of no greater than 160° C. When silicaand a coupling agent are present, the maximum discharge temperature fromthe mixing step is normally no greater than about 145° C.

For best results, the sulfur vulcanizable rubber composition containingthe sulfur containing organosilicon compound, vulcanizable rubber andgenerally at least part of the silica should be subjected to athermomechanical mixing step. The thermomechanical mixing step generallycomprises mechanical working in a mixer, mill or extruder for a periodof time suitable in order to produce a rubber temperature between 140°C. and 190° C. The appropriate duration of the thermomechanical workingvaries as a function of the operating conditions and the volume andnature of the components. For example, the thermomechanical working maybe for a duration of time which is within the range of about 1 minute toabout 20 minutes. It will normally be preferred for the rubber to reacha temperature which is within the range of about 145° C. to about 180°C.and to be maintained at said temperature for a period of time which iswithin the range of about 2 minutes to about 10 minutes. It willnormally be more preferred for the rubber to reach a temperature whichis within the range of about 155° C. to about 170° C. and to bemaintained at said temperature for a period of time which is within therange of about four minutes to about eight minutes.

The isoprene-butadiene diblock rubber containing tire tread compounds ofthis invention can be used in tire treads in conjunction with ordinarytire manufacturing techniques. Tires are built utilizing standardprocedures with the isoprene-butadiene diblock rubber simply beingsubstituted for the rubber compounds typically used as the tread rubber.After the tire has been built with the isoprene-butadiene diblock rubbercontaining blend, it can be vulcanized using a normal tire cure cycle.Tires made in accordance with this invention can be cured over a widetemperature range. However, it is generally preferred for the tires ofthis invention to be cured at a temperature ranging from about 132° C.(270° F.) to about 175° C. (347° F.). It is more typical for the tiresof this invention to be cured at a temperature ranging from about 143°C. (290° F.) to about 165° C. (329° F.). It is generally preferred forthe cure cycle used to vulcanize the tires of this invention to have aduration of about 8 to about 20 minutes with a cure cycle of about 10 to18 minutes being most preferred.

By utilizing the isoprene-butadiene diblock polymers of this inventionin tire tread compounds, treadwear characteristics can be improvedwithout compromising traction or rolling resistance. Since theisoprene-butadiene diblock polymers of this invention do not containstyrene, the cost of raw materials can also be reduced. This is becausestyrene and other vinyl aromatic monomers are expensive relative to thecost of conjugated diene monomers, such as 1,3-butadiene and isoprene.However, it is, of course, possible to include blocks which are derivedfrom vinyl aromatic monomers, such as styrene or α-methyl styrene, inthe isoprene-butadiene diblock polymer. For instance, styrene-butadieneblocks, styrene-isoprene blocks, and/or styrene-isoprene-butadieneblocks can be included in the isoprene-butadiene diblock rubber.

The isoprene-butadiene diblock rubbers of this invention can beadvantageously utilized in both automobile and truck tire treadcompounds. As a general rule, the isoprene-butadiene diblock rubberutilized in truck tire compounds will have a single glass transitiontemperature which is within the range of about -100° C. to about -70° C.On the other hand, the isoprene-butadiene diblock rubbers which areemployed in making automobile tire tread compounds will normally have afirst glass transition temperature which is within the range of about-100° C. to about -70° C. and a second glass transition temperaturewhich is within the range of about -50° C. to about 0° C..

The isoprene-butadiene diblock rubber having two glass transitiontemperatures can be blended with natural rubber to make tread compoundsfor passenger tires which exhibit outstanding rolling resistance,traction and tread wear characteristics. The utilization of naturalrubber in such blends leads to improved processability. Such blends willnormally contain from about 5 to about 30 weight percent natural rubberand from about 70 to about 95 percent of the isoprene-butadiene diblockrubber having two glass transition temperatures. Such blends willpreferably contain from about 20 weight percent to about 30 weightpercent natural rubber and about 70 to about 80 weight percent of theisoprene-butadiene diblock rubber.

High performance tires which exhibit very exceptional tractioncharacteristics, but somewhat comprised tread wear, can be prepared byblending the isoprene-butadiene diblock rubber having at least two glasstransition temperatures with solution or emulsion styrene-butadienerubber (SBR). Such blends will normally contain from about 50 weightpercent to about 75 weight percent of the isoprene-butadiene diblockpolymer and from about 25 weight percent to about 50 weight percent ofthe solution or emulsion styrene-butadiene rubber. It is typicallypreferred for such blends to contain from about 55 weight percent toabout 65 weight percent of the isoprene-butadiene diblock polymer andfrom about 35 weight percent to about 45 weight percent of the solutionor emulsion styrene-butadiene rubber.

In cases where tread wear is of greater importance than traction, fromabout 5 to about 30 weight percent high cis-1,4-polybutadiene can beblended with about 70 to about 95 weight percent of theisoprene-butadiene diblock rubber having two glass transitiontemperatures. Such blends will preferably contain from about 20 weightpercent to about 30 weight percent of the high cis-1,4-polybutadienerubber and from about 70 weight percent to about 80 weight percent ofthe isoprene-butadiene diblock rubber.

In another scenario, the isoprene-butadiene rubber of this inventionhaving essentially one glass transition temperature can be used toimprove the traction, tread wear and rolling resistance of automobiletires made therewith by including 3,4-polyisoprene in the blend. Such ablend will typically contain from about 5 to about 30 weight percent3,4-polyisoprene and from about 70 to about 95 weight percent of theisoprene-butadiene rubber having essentially one glass transitiontemperature which is within the range of about -100° C. to about -70° C.Such blends will normally contain from about 20 weight percent to about30 weight percent of the 3,4-polyisoprene and from about 70 weightpercent to about 80 weight percent of the isoprene-butadiene diblockrubber.

The 3,4-polyisoprene employed in such blends can be synthesized by thetechnique disclosed in U.S. Pat. No. 5,239,023. This technique forproducing 3,4-polyisoprene involves: (1) adding a catalyst system whichis comprised of (a) an organoiron compound which is soluble in theorganic solvent, wherein the iron in the organoiron compound is in the+3 oxidation state, (b) a partially hydrolyzed organoaluminum compound,which was prepared by adding a protonic compound selected from the groupconsisting of water, alcohols and carboxylic acids to the organoaluminumcompound, and (c) a chelating aromatic amine; wherein the molar ratio ofthe chelating amine to the organoiron compound is within the range ofabout 0.1:1 to about 1:1, wherein the molar ratio of the organoaluminumcompound to the organoiron compound is within the range of about 5:1 toabout 200:1, and wherein the molar ratio of the protonic compound to theorganoaluminum compound is within the range of about 0.001:1 to about0.2:1 to a polymerization medium containing isoprene monomer and anorganic solvent, and (2) allowing the isoprene monomer to polymerize ata temperature which is within the range of about -10° C. to about 100°C. Another representative example of a 3,4-polyisoprene rubber which canbe employed in the automobile tire tread compounds of this invention issold by Huels AG under the tradename Vestogrip® A6001.

Truck tire tread compounds are typically prepared by blending from about5 to about 30 weight percent of natural rubber and/or highcis-1,4-polybutadiene with about 70 to about 95 weight percent of thesingle glass transition temperature version of the isoprene-butadienediblock rubber. High cis-1,4-polybutadiene which is suitable for use insuch blends can be made by the process described in Canadian Patent1,236,648. High cis-1,4-polybutadiene rubber which is suitable foremployment in such blends is also sold by The Goodyear Tire & RubberCompany as Budene® 1207 polybutadiene rubber and Budene® 1208polybutadiene rubber.

Treads for high performance tires can also be made by blending fromabout 30 weight percent to about 80 weight percent of theisoprene-butadiene diblock rubber with about 20 weight percent to about70 weight percent of high vinyl polybutadiene rubber having a vinylcontent of 60 to about 90 percent. Better traction characteristics cannormally be realized by incorporation a higher level of high vinylpolybutadiene rubber into the blend. It is accordingly normallypreferred to blend from about 50 weight percent to about 70 weightpercent of the isoprene-butadiene diblock rubber with about 30 weightpercent to about 50 weight percent of the high vinyl polybutadienerubber. It is generally more preferred to blend from about 55 weightpercent to about 65 weight percent of the isoprene-butadiene diblockrubber with about 35 weight percent to about 45 weight percent of thehigh vinyl polybutadiene rubber. The high vinyl polybutadiene rubberwill typically have a vinyl content which is within the range of about60 percent to about 80 percent.

Treads for high performance tires can also be made by blending mediumvinyl polybutadiene rubber with the isoprene-butadiene rubber in caseswhere better rolling resistance is demanded. The medium vinylpolybutadiene rubber utilized in such cases has a vinyl content which iswithin the range of about 30 percent to 59 percent. The medium vinylpolybutadiene rubber preferably has a vinyl content which is within therange of about 40 percent to about 50 percent. For instance, treads forhigh performance tires can be made by blending from about 30 weightpercent to about 80 weight percent of the isoprene-butadiene diblockrubber with about 20 weight percent to about 70 weight percent of mediumvinyl polybutadiene rubber. It is normally preferred to blend from about50 weight percent to about 70 weight percent of the isoprene-butadienediblock rubber with about 30 weight percent to about 50 weight percentof the medium vinyl polybutadiene rubber. It is generally more preferredto blend from about 55 weight percent to about 65 weight percent of theisoprene-butadiene diblock rubber with about 35 weight percent to about45 weight percent of the medium vinyl polybutadiene rubber.

Treads for high performance automobile tires can also be made byblending styrene-isoprene-butadiene rubber (SIBR) with theisoprene-butadiene diblock rubber. Such blends will normally containfrom 30 weight percent to about 80 weight percent of theisoprene-butadiene diblock rubber and from about 20 to about 70 weightpercent of the SIBR. It is normally preferred to blend from about 50weight percent to about 70 weight percent of the isoprene-butadienediblock rubber with about 30 weight percent to about 50 weight percentof the SIBR. It is generally more preferred to blend from about 55weight percent to about 65 weight percent of the isoprene-butadienediblock rubber with about 35 weight percent to about 45 weight percentof the SIBR. The SIBR utilized in such tire tread compounds willtypically have a glass transition temperature which is within the rangeof about -40° C. to about -20° C.

For purposes of this patent application, polymer microstructures aredetermined by nuclear magnetic resonance spectrometry (NMR). Glasstransition temperatures (Tg) are determined by differential scanningcalorimetry at a heating rate of 10° C. per minute and molecular weightsare determined by gel permeation chromatography (GPC).

This invention is illustrated by the following examples which are merelyfor the purpose of illustration and are not to be regarded as limitingthe scope of the invention or the manner in which it can be practiced.Unless specifically indicated otherwise, all parts and percentages aregiven by weight.

EXAMPLES 1-3

In this series of experiments, low Tg/high Tg isoprene-butadiene diblockrubbery elastomers were prepared utilizing the techniques of thisinvention. The rubbers synthesized in this series of experiments werecomprised of a first segment consisting of repeat units which werederived from 1,3-butadiene and a second segment which consisted ofrepeat units deriving from isoprene and 1,3-butadiene.

The diblock polymers prepared in this series of experiments weresynthesized in a one-gallon reactor (3.8 liter) batch polymerizationreactor. In the procedure used, 509 grams of a premix solutioncontaining 19.6 percent 1,3-butadiene monomer in hexane was charged intothe polymerization reactor. Polymerization was initiated by the additionof 2.1 ml of a 1.02M solution of n-butyllithium (0.15 ml out of this 2.1ml n-butyllithium was used to scavenge the impurities contained in thepremix). The reactor was maintained at a temperature of about 65° C.until essentially complete conversion had been achieved.

At this point, 7.4 ml of a 1.05M solution of ethyl tetrahydrofurfurylether (ETE) in hexane was added to reactor. Then, 1,500 grams of ascavenged premix solution containing 19.95 percent isoprene and1,3-butadiene in hexane was added. The premix monomer solution containeda ratio of isoprene to 1,3-butadiene of 50:50. The polymerization wascontinued at 65° C. until an essentially complete conversion wasattained. Three ml of 1M ethanol solution (in hexane) was added to thereactor to shortstop the polymerization and polymer was removed from thereactor and stabilized with 1 phm of an antioxidant. After evaporatinghexane, the resulting polymer was dried in a vacuum oven at 50° C.. Theratio of the two segments in this polymer was 25:75. The diblock rubberswith other segment ratios were prepared similarly and are shown in TableI.

The three diblock rubbers synthesized in this series of experimentsdisplayed two glass transition temperatures which were within the rangesof about -94° C. to about -95° C. and about -23° C. and -24° C. Themicrostructure of the diblock rubbers is also shown in Table I.

                                      TABLE I                                     __________________________________________________________________________         Segment                                                                            Tg                                                                  Example                                                                            Ratio                                                                              (°C.)                                                                       ML-4                                                                             1,2-PBd                                                                           1,4-PBd                                                                           1,2-PI                                                                            3,4-PI                                                                            1,4-PI                                      __________________________________________________________________________    1    25:75                                                                              -94, -24                                                                           100                                                                              30  32  3   28  7                                           2    50:50                                                                              -95, -23                                                                           86 21  52  2   19  6                                           3    75:25                                                                              -94, -23                                                                           106                                                                              14  74  0   8   4                                           __________________________________________________________________________

EXAMPLES 4-6

The procedure described in Examples 1-3 was utilized in these examplesexcept that the ratio of isoprene to 1,3-butadiene in the second monomerpremix was changed from 50:50 to 30:70 and no ETE modifier was used tocomplete the polymerization of monomers for the second segment of thediblock rubbers. The three diblock rubbers synthesized in this series ofexperiments displayed only one glass transition temperature which waswithin -89° C. and -94° C. The Tgs, Mooney ML-4 viscosities at 100° C.,and microstructures of the resulting diblock rubbers are listed in TableII.

                                      TABLE II                                    __________________________________________________________________________         Segment                                                                            Tg                                                                  Example                                                                            Ratio                                                                              (°C.)                                                                      ML-4                                                                              1,2-PBd                                                                           1,4-PBd                                                                           1,2-PI                                                                            3,4-PI                                                                            1,4-PI                                      __________________________________________________________________________    4    25:75                                                                              -89 54  7   69  0   1   23                                          5    50:50                                                                              -90 53  7   77  0   1   15                                          6    75:25                                                                              -94 51  8   83  0   0   8                                           __________________________________________________________________________

EXAMPLE 7

The diblock 50/50 PBD-(30/70) IBR prepared in this experiment wassynthesized in a two-reactor (20 liters for the first reactor and 40liters for the second reactor) continuous system at 90° C. A premixcontaining 14 percent 1,3-butadiene in hexane was charged into the firstpolymerization reactor continuously at a rate of 150 grams/minute.Polymerization was initiated by adding a 0.207M solution ofn-butyllithium into the first reactor at a rate of 0.32 grams/minute.

The polymerization medium was continuously pushed over from the firstreactor to the second reactor where the second premix monomer solutionwas added at a rate of 150 grams/minute. The second premix monomersolution contained a ratio of isoprene to 1,3-butadiene of 30:70 and hada total monomer concentration of 14 percent in hexane. The temperatureof the second reactor was also maintained at 90° C. The residence timefor both reactors was set at 1.5 hours. The average monomer conversionswere determined to be 94 percent for the first reactor and 97 percentfor the second reactor.

The polymerization medium was then continuously pushed over to a holdingtank which contained isopropanol (as a shortstop) and an antioxidant.The resulting polymer cement was then steam-stripped and the diblockrubber recovered was dried in a vacuum over at a temperature of 50° C.The polymer was determined to have a glass transition temperature at-89° C. and have a Mooney ML-4 viscosity at 100° C. of 73. It was alsodetermined to have a microstructure which contained 10 percent1,2-polybutadiene units, 73 percent 1,4-polybutadiene units, 15 percent1,4-polyisoprene units and 2 percent 1,2-polyisoprene units.

EXAMPLES 8-9

The procedure described in Example 7 was utilized in these experimentsto synthesize the diblock low Tg/high Tg IBR-IBRs except that the firstpremix solution was changed from 1,3-butadiene to a mixture of isopreneand 1,3-butadiene and also a mixed modifiers,N,N,N',N'-tetramethylethylene (TMEDA)/sodium-t-amylate (STA) was chargedto the second reactor at the TMEDA to STA and to n-butyl-lithium molarratio of 3:0.5:1. The two diblock rubbers prepared in this series ofexperiments displayed two glass transition temperatures which werewithin the ranges of about -77° C. to about -83° C. and about -15° C. to-23° C. The compositions of each segment in these diblock rubbers andtheir glass transition temperatures, Mooney ML-4 viscosities at 100° C.and microstructures are shown in Table III.

                                      TABLE III                                   __________________________________________________________________________           Composition                                                                   1st 2nd                                                                Segment                                                                              segment Tg   Mooney                                                    Ex                                                                              Ratio                                                                              Isop/Bd                                                                           Isop/Bd                                                                           (°C.)                                                                       ML-4                                                                              1,2-PBd                                                                           1,4-PBd                                                                           1,2-PI                                                                            3,4-P1                                                                            1,4-PI                                __________________________________________________________________________    8 50:50                                                                              30/70                                                                             30/70                                                                             -83, -23                                                                           70  30  44  0   8   18                                    9 50:50                                                                              50/50                                                                             50/50                                                                             -77, -15                                                                           51  21  30  4   17  28                                    __________________________________________________________________________

EXAMPLES 10-14

The isoprene-butadiene diblock rubbers made in Examples 1 and 6 werethen compounded utilizing a standard tire tread test formulation andcompared to tire tread formulations made with solution styrene-butadienerubber, styrene-isoprene-butadiene rubber, and a 50%/50% blend ofnatural rubber and styrene-butadiene rubber. The tire tread testformulations were made by mixing 100 parts of the rubber being testedwith 45 parts of carbon black, 9 parts of process oil, 3 parts ofstearic acid, 3 parts of zinc oxide, 1 part of microcrystalline wax, 0.5parts of paraffine wax, 1 part of a mixed aryl-p-phenylenediamineantioxidant, 2 parts of N-(1,3-dimethyl butyl)-N'-phenyl-p-phenylenediamine, 0.8 parts of N-oxydiethylene benzothiazole-2-sulfenamide, 0.4parts of diphenyl guanidine and 1.6 parts of sulfur. In Example 10, theisoprene-butadiene diblock rubber made in Example 1 was included in theformulation and, in Example 11, the isoprene-butadiene diblock rubbermade in Example 2 was included in the formulation. Examples 12-14 werecarried out as comparative examples and included styrene-butadienerubber, styrene-isoprene-butadiene rubber and the 50%/50% blend ofnatural rubber and styrene-butadiene rubber, respectively, as the rubbercomponent.

The physical properties of the compounded tire tread formulations arereported in Table IV.

                  TABLE IV                                                        ______________________________________                                        Compound Physical Properties                                                  Example     10      11      12    13    14                                    ______________________________________                                        Rubber Component                                                                          IBR     IBR     SBR   SIBR  NR/SBR                                Rheometer, 150°C.                                                      ML, dNm     4.4     3.2     4.0   2.7   2.6                                   MH, dNm     19.8    21.6    22.8  17.8  15.8                                  ts1, min.   5.7     5.7     9.1   7.7   5.9                                   T25, min.   8.4     8.6     12.5  10.4  7.7                                   T90, min.   16.9    13.6    20.3  18.8  14.7                                  Stress-Strain,                                                                18'/150° C.                                                            100% Modulus, MPa                                                                         2.0     2.4     2.1   1.8   1.8                                   300% Modulus, MPa                                                                         9.4     9.3     8.2   8.8   9.0                                   Break Strength, MPa                                                                       16.4    11.7    17.0  16.1  19.9                                  Elongation at Break                                                                       473%    380%    558%  515%  554%                                  Rebound                                                                       rebound at 23° C.                                                                   36%     64%     52%   31%   59%                                  rebound at 100° C.                                                                  65%     71%     63%   64%   66%                                  DIN Abrasion, cc.sup.1                                                                    126     28      71    179   123                                   Autovibron, 11 Hz                                                             tan delta at 0° C.                                                                 .392    .076    .117  .368  .163                                  tan delta at 60° C.                                                                .065    .048    .096  .100  .084                                  ______________________________________                                         .sup.1 Reported in cubic centimeters of volume loss.                     

Table IV shows that the isoprene-butadiene diblock rubbers of thisinvention exhibit low tan delta values at 60° C. while exhibiting veryhigh tan delta values at 0° C. Low tan delta values at 60° C. areindicative of good rolling resistance when incorporated into tire treadsand high tan delta values at 0° C. are indicative of good tractioncharacteristics. Accordingly, tire treads can be made with theisoprene-butadiene diblock rubbers of this invention which have bothimproved traction characteristics and rolling resistance. Example 10depicts an excellent tire tread compound for automobile tires which willprovide outstanding traction, tread durability and rolling resistance.This is because it exhibits a tan delta at 0° C. of greater than 0.35while displaying a tan delta at 60° C. of less than 0.070. Suchcompounds would, of course, be highly desirable in high performancetires.

Such compounds which exhibit large differences between the tan deltavalue at 0° C. and their tan delta value at 60° C. offer an array ofadvantages in tire tread compounding applications. For instance, it isgenerally considered to be good for the difference between tan delta at0° C. and tan delta at 60° C. to be 0.150 or greater. It is excellentfor the difference between tan delta at 0° C. and the tan delta at 60°C. to be 0.2 or greater and it is very exceptional for this differencein tan delta values to be greater than 0.25. In the case of the compoundmade in Example 10, the difference between tan delta at 0° C. and tandelta at 60° C. is greater than 0.30.

The tire tread compound depicted in Example 11 could be used in trucktires to provide exceptional rolling resistance and tread durabilitywith somewhat compromised traction characteristics. In the case of trucktires, traction characteristics are generally not of great concernbecause of very heavy vehicle weights. Thus, the compound made inExample 11 has good characteristics for truck tires. In any case, thecompound depicted in Example 11 displays a tan delta at 60° C. of lessthan 0.050 which is indicative of superb rolling resistance andtreadwear characteristics. As can be seen, the tan delta attained at 60°in Example 11 is less than that realized in any of the controlcompounds. The abrasion resistance observed in Example 11 wasoutstanding with the DIN abrasion being less than 30 cc. A DIN abrasionof less than 50 cc is considered to be excellent and a DIN abrasion ofless than 40 cc is considered to be superb for tire treadwear.

EXAMPLES 15-19

The isoprene-butadiene diblock rubbers made in Examples 8 and 9 werethen compounded utilizing a tire tread test formulation and compared totire tread formulations made with a blend of emulsion styrene-butadienerubber and high cis-1,4-polybutadiene rubber. The tire tread testformulations were made by mixing the ingredients shown in Table V.Example 15 was carried out as a comparative example and did not includeany of the isoprene-butadiene diblock rubber of this invention, as therubber component.

                  TABLE V                                                         ______________________________________                                        Example   15       16      17     18    19                                    ______________________________________                                        Emulsion SBR.sup.1                                                                      96.3     --      --     --    --                                    Cis-1,4-PBD.sup.2                                                                       37.5     --      37.5   --    37.5                                  IBR (Example 8)                                                                         --       100     70     --    --                                    IBR (Example 9)                                                                         --       --      --     100   70                                    carbon black                                                                            93       93      93     93    93                                    wax       4        4       4      4     4                                     zinc oxide                                                                              4        4       4      4     4                                     stearic acid                                                                            2        2       2      2     2                                     CBS       3.5      3.5     3.5    3.5   3.5                                   TMTD      0.25     0.25    0.25   0.25  0.25                                  sulfur    0.85     0.85    0.85   0.85  0.85                                  Processing Oil                                                                          --       33.8    26.3   33.8  26.3                                  Antidegradant                                                                           1.5      1.5     1.5    1.5   1.5                                   ______________________________________                                         .sup.1 The 96.3 parts of emulsion styrenebutadiene rubber contained 70        parts of rubber and 26.3 parts of processing oil. The emulsion                styrenebutadiene rubber contained 23.5% bound styrene.                        .sup.2 The 37.5 parts of high cis1,4-polybutadiene rubber contained 30        parts of rubber and 7.5 parts of processing oil.                         

The physical properties of the compounded tire tread formulations arereported in Table VI.

                  TABLE VI                                                        ______________________________________                                        Compound Physical Properties                                                  Example     15      16      17    18    19                                    ______________________________________                                        Rubber Component                                                                          SBR/    IBR     IBR/  IBR   IBR/                                              PBD             PBD         PBD                                   Rheometer, 150°C.                                                      ML, dNm     9.1     8.9     9.2   6.3   8.0                                   MH, dNm     34.8    33.2    34.1  25.8  28.0                                  ts1, min.   7.5     6.2     6.2   7.0   6.5                                   T25, min.   10.0    9.0     7.8   8.0   7.5                                   T90, min.   14.0    10.7    10.0  11.0  9.6                                   Stress-Strain,                                                                18'/150° C.                                                            100% Modulus, MPa                                                                         1.85    1.85    1.78  1.78  1.56                                  300% Modulus, MPa                                                                         7.47    7.01    6.65  6.37  5.57                                  Break Strength, MPa                                                                       15.8    12.1    12.3  9.3   9.6                                   Elongation at Break                                                                       596%    516%    534%  461%  478%                                  Rebound                                                                       rebound at 23° C.                                                                   24%     24%     27%   19%   22%                                  rebound at 100° C.                                                                  39%     39%     41%   34%   35%                                  DIN Abrasion, cc                                                                          113     135     104   200   145                                   Autovibron, 11 Hz                                                             tan delta at 0° C.                                                                 .103    .105    .098  .182  .122                                  tan delta at 60° C.                                                                .145    .140    .139  .154  .143                                  ______________________________________                                    

EXAMPLES 20-22

The isoprene-butadiene diblock rubbers made in Example 7 was compoundedutilizing two different tire tread test formulations and compared to atire tread formulation made with a blend of emulsion styrene-butadienerubber and high cis-1,4-polybutadiene rubber. The tire tread testformulations were made by mixing the ingredients shown in Table VII.Example 20 was carried out as a comparative example and did not includedany of the isoprene-butadiene diblock rubber of this invention, as therubber component.

                  TABLE VII                                                       ______________________________________                                        Example          20         21     22                                         ______________________________________                                        IBR (Example 7)  --         70     55                                         3,4-polyisoprene --         --     15                                         Emulsion SBR.sup.1                                                                             96.25      --     --                                         High cis-1,4-polybutadiene.sup.2                                                               37.5       37.5   37.5                                       Process Oil      10         36.25  36.25                                      Carbon Black     70         70     70                                         Zinc Oxide       2          2      2                                          Wax              4          4      4                                          Stearic Acid     2          2      2                                          CBS              1          1      1                                          TMTD             0.3        0.3    0.3                                        Sulfur           1.5        1.5    1.5                                        Wingstay ® 100 Antioxidant                                                                 1          1      1                                          ______________________________________                                         .sup.1 The 96.25 parts of emulsion styrenebutadiene rubber contained 70       parts of rubber and 26.25 parts of processing oil. The emulsion               styrenebutadiene rubber contained 23.5% bound styrene.                        .sup.2 The 37.5 parts of high cis1,4-polybutadiene rubber contained 30        parts of rubber and 7.5 parts of processing oil. The high                     cis1,4-polybutadiene was Budene ® 1254 polybutadiene rubber.         

The physical properties of the compounded tire tread formulations arereported in Table VIII.

                  TABLE VIII                                                      ______________________________________                                        Compound Physical Properties                                                  Example       15         16      17                                           ______________________________________                                        Rubber Component                                                                            SBR/PBD    IBR     IBR/3,4-PI                                   Rheometer, 150°C.                                                      ML, dNm       2.9        3.2     3.1                                          MH, dNm       13.4       16.7    16.2                                         ts1, min.     5.6        4.8     4.6                                          T25, min.     6.5        5.3     5.1                                          T90, min.     15.5       8.9     8.5                                          Stress-Strain,                                                                18'/150° C.                                                            100% Modulus, MPa                                                                           1.2        1.3     1.4                                          300% Modulus, MPa                                                                           4.3        3.9     4.4                                          Elongation at Break                                                                         785%       639%    630%                                         Rebound                                                                       rebound at 23° C.                                                                     32%        41%     35%                                         rebound at 100° C.                                                                    46%        51%     51%                                         DIN Abrasion, cc                                                                            107        81      91                                           Autovibron, 11 Hz                                                             tan delta at 0° C.                                                                   .122       .103    .186                                         tan delta at 60° C.                                                                  .132       .106    .107                                         ______________________________________                                    

EXAMPLE 19

In this experiment, an isoprene-butadiene diblock polymer having a firstblock which was comprised of repeat units which were derived fromisoprene and 1,3-butadiene and a second block which was also comprisedof repeat units which were derived from isoprene and 1,3-butadiene wassynthesized. The first isoprene-butadiene block in the polymer made hada low vinyl content and the second block had a high vinyl content.

In this experiment, ethyl tetrahydrofurfuryl ether (ETE) was employed asthe modifier. In the procedure utilized, 830 grams of a silica/molecularsieve/alumina dried premix containing isoprene and 1,3-butadiene inhexane was charged into a one-gallon (3.8 liter) reactor. The premixmonomer solution contained a ratio of isoprene to 1,3-butadiene of 50:50and the total monomer concentration was 18.2 percent. The monomer premixsolution had been previously scavenged for impurities with an-butyllithium solution. Polymerization was initiated by the addition of1.6 ml of a 1.04M solution of n-butyllithium.

The reactor was maintained at a temperature of about 65° C. untilessentially complete monomer conversion had been achieved which tookabout 2.5 hours. Then, 4.2 ml of a 1.0M solution of ETE was added to thepolymerization medium and followed by an additional 1620 grams ofscavenged monomer premix (the premix had a 50:50 ratio of isoprene to1,3-butadiene and a concentration of 18.2 percent in hexane). Thecopolymerization was allowed to continue at 65° C. until all themonomers were consumed which took about two hours. The polymerizationwas shortstopped by the addition ethanol to the polymerization mediumand the isoprene-butadiene diblock rubber synthesized was stabilizedwith 1 phr (parts per hundred parts of rubber) of antioxidant. Afterevaporating the hexane solvent, the resulting isoprene-butadiene diblockrubber was dried in a vacuum oven at a temperature of 50° C.

The isoprene-butadiene diblock rubber made was determined to have twoglass transition temperatures at -80° C. and -31° C. The rubber made wasalso determined to have a microstructure which contained 24%1,2-polybutadiene units, 27% 1,4-polybutadiene units, 24 percent3,4-polyisoprene units, 24 percent 1,4-polyisoprene units and 1 percent1,2-polyisoprene units.

Variations in the present invention are possible in light of thedescription of it provided herein. While certain representativeembodiments and details have been shown for the purpose of illustratingthe subject invention, it will be apparent to those skilled in this artthat various changes and modifications can be made therein withoutdeparting from the scope of the subject invention. It is, therefore, tobe understood that changes can be made in the particular embodimentsdescribed which will be within the full intended scope of the inventionas defined by the following appended claims.

What is claimed is:
 1. A pneumatic tire having an outer circumferentialtread wherein said tread is a sulfur-cured rubber composition comprisedof, based on 100 parts by weight of rubber, (a) from about 30 to about80 parts of an isoprene-butadiene diblock rubber, saidisoprene-butadiene diblock rubber being comprised of a butadiene blockand an isoprene-butadiene block, wherein said butadiene block has anumber average molecular weight which is within the range of about25,000 to about 350,000, wherein said isoprene-butadiene block has anumber average molecular weight which is within the range of about25,000 to about 350,000, wherein said isoprene-butadiene diblock rubberhas essentially one glass transition temperature which is within therange of about -100° C. to about -70° C., wherein saidisoprene-butadiene diblock rubber has a Mooney ML-4 viscosity at 100° C.which is within the range of about 50 to about 140, wherein saidisoprene-butadiene diblock rubber does not contain any blocks which arederived solely from isoprene, and wherein the repeat units derived fromisoprene and 1,3-butadiene in the isoprene-butadiene block are inessentially random order; and (b) from about 20 to about 70 parts of asecond rubber selected from the group consisting of high vinylpolybutadiene rubber, medium vinyl polybutadiene rubber, andstyrene-isoprene-butadiene rubber.
 2. A pneumatic tire having an outercircumferential tread wherein said tread is a sulfur-cured rubbercomposition comprised of, based on 100 parts by weight of rubber, (a)from about 30 to about 80 parts of an isoprene-butadiene diblock rubber,said isoprene-butadiene diblock rubber being comprised of a butadieneblock and an isoprene-butadiene block, wherein said butadiene block hasa number average molecular weight which is within the range of about25,000 to about 350,000, wherein said isoprene-butadiene block has anumber average molecular weight which is within the range of about25,000 to about 350,000, wherein said isoprene-butadiene diblock rubberhas a first glass transition temperature which is within the range ofabout -100° C. to about -70° C., wherein said isoprene-butadiene diblockrubber has a second glass transition temperature which is within therange of about -50° C. to about 0° C., wherein said isoprene-butadienediblock polymer has a Mooney ML-4 at 100° C. viscosity which is withinthe range of about 50 to about 140, wherein said isoprene-butadienediblock rubber does not contain any blocks which are derived solely fromisoprene, and wherein the repeat units derived from isoprene and1,3-butadiene in the isoprene-butadiene block are in essentially randomorder; and (b) from about 20 to about 70 parts of a second rubberselected from the group consisting of high vinyl polybutadiene rubberand styrene-isoprene-butadiene rubber.
 3. A pneumatic tire having anouter circumferential tread wherein said tread is a sulfur-cured rubbercomposition comprised of, based on 100 parts by weight of rubber, (a)from about 50 to about 75 parts of an isoprene-butadiene diblock rubber,said isoprene-butadiene diblock rubber being comprised of a butadieneblock and an isoprene-butadiene block, wherein said butadiene block hasa number average molecular weight which is within the range of about25,000 to about 350,000, wherein said isoprene-butadiene block has anumber average molecular weight which is within the range of about25,000 to about 350,000, wherein said isoprene-butadiene diblock rubberhas essentially one glass transition temperature which is within therange of about -100° C. to about -70° C., wherein saidisoprene-butadiene diblock polymer has a Mooney ML-4 viscosity at 100°C. which is within the range of about 50 to about 140, wherein saidisoprene-butadiene diblock rubber does not contain any blocks which arederived solely from isoprene, and wherein the repeat units derived fromisoprene and 1,3-butadiene in the isoprene-butadiene block are inessentially random order; and (b) from about 25 to about 50 parts of asecond rubber selected from the group consisting of emulsionstyrene-butadiene rubber and solution styrene-butadiene rubber.
 4. Apneumatic tire having an outer circumferential tread wherein said treadis a sulfur-cured rubber composition comprised of, based on 100 parts byweight of rubber, (a) from about 50 to about 75 parts of anisoprene-butadiene diblock rubber, said isoprene-butadiene diblockrubber being comprised of a butadiene block and an isoprene-butadieneblock, wherein said butadiene block has a number average molecularweight which is within the range of about 25,000 to about 350,000,wherein said isoprene-butadiene block has a number average molecularweight which is within the range of about 25,000 to about 350,000,wherein said isoprene-butadiene diblock rubber has a first glasstransition temperature which is within the range of about -100° C. toabout -70° C., wherein said isoprene-butadiene diblock rubber has asecond glass transition temperature which is within the range of about-50° C. to about 0° C., wherein said isoprene-butadiene diblock polymerhas a Mooney ML-4 at 100° C. viscosity which is within the range ofabout 50 to about 140, wherein said isoprene-butadiene diblock rubberdoes not contain any blocks which are derived solely from isoprene, andwherein the repeat units derived from isoprene and 1,3-butadiene in theisoprene-butadiene block are in essentially random order; and (b) fromabout 25 to about 50 parts of a second rubber selected from the groupconsisting of emulsion styrene-butadiene rubber and solutionstyrene-butadiene rubber.
 5. A pneumatic tire as specified in claim 1wherein the second rubber is high vinyl polybutadiene having a vinylcontent which is within the range of about 60 percent to about 80percent.
 6. A pneumatic tire as specified in claim 1 wherein the secondrubber is a styrene-isoprene-butadiene rubber having a glass transitiontemperature which is within the range of about -40° C. to about -20° C.7. A pneumatic tire as specified in claim 5 wherein said tread iscomprised of about 50 weight percent to about 70 weight percent of theisoprene-butadiene diblock rubber and from about 30 weight percent toabout 50 weight percent of the high vinyl polybutadiene rubber.
 8. Apneumatic tire as specified in claim 7 wherein said isoprene-butadienediblock rubber has a Mooney viscosity at 100° C. which is within therange of about 80 to about 135; and wherein the butadiene block has anumber average molecular weight which is within the range of about50,000 to about 200,000; and wherein the isoprene-butadiene block has anumber average molecular weight which is within the range of about50,000 to about 200,000.
 9. A pneumatic tire as specified in claim 8wherein the isoprene-butadiene block in the isoprene-butadiene diblockpolymer is comprised of from about 10 weight percent to about 60 weightpercent repeat units which are derived from isoprene, and from about 40weight percent to about 90 weight percent repeat units which are derivedfrom 1,3-butadiene; wherein the ratio of the number average molecularweight of the butadiene block to the number average molecular weight ofthe isoprene-butadiene block is within the range of about 25/75 to about75/25.
 10. A pneumatic tire as specified in claim 9 wherein thebutadiene block in the isoprene-butadiene diblock polymer has a numberaverage molecular weight which is within the range of about 70,000 toabout 150,000; and wherein the isoprene-butadiene block in theisoprene-butadiene diblock polymer has a number average molecular weightwhich is within the range of about 70,000 to about 150,000.
 11. Apneumatic tire as specified in claim 10 wherein the isoprene-butadieneblock in the isoprene-butadiene diblock polymer is comprised of fromabout 20 weight percent to about 50 weight percent repeat units whichare derived from isoprene, and from about 50 weight percent to about 80weight percent repeat units which are derived from 1,3-butadiene; andwherein the ratio of the number average molecular weight of thebutadiene block to the number average molecular weight of theisoprene-butadiene block is within the range of about 35/65 to about65/35.
 12. A pneumatic tire as specified in claim 11 wherein theisoprene-butadiene block is comprised of from about 30 weight percent toabout 45 weight percent repeat units which are derived from isoprene,and from about 55 weight percent to about 70 weight percent repeat unitswhich are derived from 1,3-butadiene; wherein said isoprene-butadienediblock rubber has a Mooney viscosity at 100° C. which is within therange of about 100 to about
 130. 13. A pneumatic tire as specified inclaim 12 wherein said tread is comprised of about 55 weight percent toabout 65 weight percent of the isoprene-butadiene diblock rubber andfrom about 35 weight percent to about 45 weight percent of the highvinyl polybutadiene rubber.
 14. A pneumatic tire as specified in claim13 wherein said tread is a sulfur-cured rubber composition which isfurther comprised of carbon black, at least one antidegradant, at leastone processing oil and zinc oxide.
 15. A pneumatic tire as specified inclaim 13 wherein said tread is a sulfur-cured rubber composition whichis further comprised of silica.
 16. A pneumatic tire as specified inclaim 6 wherein said tread is comprised of about 50 weight percent toabout 70 weight percent of the isoprene-butadiene diblock rubber andfrom about 30 weight percent to about 50 weight percent of thestyrene-isoprene-butadiene rubber.
 17. A pneumatic tire as specified inclaim 3 wherein the second rubber is solution styrene-butadiene rubber.18. A pneumatic tire as specified in claim 17 wherein the tread iscomprised of about 55 weight percent to about 65 weight percent of theisoprene-butadiene diblock rubber and from about 35 weight percent toabout 45 weight percent of the styrene-isoprene-butadiene rubber.
 19. Apneumatic tire as specified in claim 4 wherein the second rubber issolution styrene-butadiene rubber; and wherein the tread is comprised ofabout 55 weight percent to about 65 weight percent of theisoprene-butadiene diblock rubber and from about 35 weight percent toabout 45 weight percent of the styrene-isoprene-butadiene rubber.
 20. Apneumatic tire as specified in claim 2 wherein the second rubber is highvinyl polybutadiene having a vinyl content which is within the range ofabout 60 percent to about 80 percent; and wherein said tread iscomprised of about 50 weight percent to about 70 weight percent of theisoprene-butadiene diblock rubber and from about 30 weight percent toabout 50 weight percent of the high vinyl polybutadiene rubber.
 21. Apneumatic tire as specified in claim 1 wherein the second rubber ismedium vinyl polybutadiene rubber having a vinyl content of 40 percentto 50 percent.
 22. A pneumatic tire as specified in claim 1 wherein saidtread is a sulfur-cured rubber composition which is further comprised ofsilica.
 23. A pneumatic tire as specified in claim 22 wherein the silicais present in an amount which is within the range of about 10 phr toabout 250 phr.
 24. A pneumatic tire as specified in claim 22 wherein thesilica is present in an amount which is within the range of about 50 phrto about 120 phr.
 25. A pneumatic tire as specified in claim 24 whereinthe rubber composition is further comprised of a silica coupling agent.26. A pneumatic tire as specified in claim 25 wherein the silica isadded by a thermomechanical mixing step.
 27. A pneumatic tire asspecified in claim 26 wherein the silica coupling agent isbis-(3-triethoxysilylpropyl)tetrasulfide.